A power supply controller is conventionally provided, in which a high-power semiconductor switching element such as a power MOSFET is disposed on a current supply line connected between a power source and a load, and which is configured to control the power supply to the load by switching the semiconductor switching element between ON and OFF. It is known that some of such power supply controllers have a self-protective function. When an overcurrent (i.e., an abnormal current) has occurred on the current supply line due to short-circuiting in the load, for example, the self-protective function turns off the semiconductor switching element by controlling the potential of the control terminal (e.g., the gate in the case of a MOSFET) of the semiconductor switching element, in order to protect the semiconductor switching element. Specifically, as shown in JP-A-2001-217696, a current detecting resistor (shunt resistor) is serially connected to the load terminal (e.g., the source or drain in the case of a MOSFET) of the semiconductor switching element, and the value of a load current passing through the semiconductor switching element is detected based on the interterminal voltage of the resistor. If the load current value is larger than a predetermined threshold, an occurrence of an overcurrent anomaly is determined, so as to turn off the semiconductor switching element resulting in a shutoff state.
A current passing through the semiconductor switching element will vary along a predetermined load line, until a stable state is reached after start-up of the semiconductor switching element. Therefore, in the case that an overcurrent anomaly is detected based on comparison of the load current value of the semiconductor switching element with the threshold, there arises a problem that it may require time before detection when an overcurrent anomaly has occurred, if the threshold is set to a fixed level. For example, FIG. 17 shows the drain-to-source voltage Vds of a power MOSFET as the above semiconductor switching element and the current Id passing therethrough. If the load is in a normal state, after the power MOSFET turns ON, the values of the drain-to-source voltage Vds and the current Id will vary along the load line L0 starting from the point B0 resulting in settlement at the stabilization point A0.
However, in case that an anomaly such as short-circuiting in the load has occurred, although the values of the drain-to-source voltage Vds and the current Id can start from the point B0 on start-up, the source voltage of the power MOSFET will thereafter rise very little because the voltage drop in the load is extremely low. That is, the current Id passing through the power MOSFET will rise steeply while the drain-to-source voltage Vds of the power MOSFET varies little. If the threshold is set to a fixed level, i.e., if one threshold value is used for detection of any overcurrent anomaly capable of occurrence at some point between start-up of the power MOSFET and a stable state being reached, the threshold should be set to a fixed level higher than the current value Id of the stabilization point A0 (as shown by Line L7 in the figure) so that an overcurrent anomaly can be detected even after the stabilization point A0 is reached. In this case, if short-circuiting occurs immediately after the power MOSFET turns ON, it requires considerable time before the threshold is reached as shown by Line L6. This will result in great power loss in the power MOSFET, and cause delay in protection.
In view of the above, the present applicant developed an invention and already applied for patent (Japanese Patent Application No. 2005-163967), according to which a voltage corresponding to the output-side voltage (e.g., the source voltage or drain voltage in the case of a MOSFET) of a semiconductor switching element is generated so that an overcurrent anomaly can be detected based on whether a load current passing through the semiconductor switching element exceeds a threshold value corresponding to the generated voltage. According to this construction, the threshold can be set so as to increase or decrease with the output-side voltage of the semiconductor switching element. Thereby, in case that short-circuiting has occurred in the load, for example, the load current will immediately reach the threshold, so that rapid protection can be achieved compared to a construction in which a threshold is set to a fixed level.
However, in the above construction, the threshold will be accordingly set to a low value when the semiconductor switching element is in a stable state after start-up but the power supply voltage is low, because the threshold is changed according to the output-side voltage of the semiconductor switching element. In this case, an overcurrent anomaly may be determined resulting in turn-off of the semiconductor switching element, even when the power loss in the semiconductor switching element is much lower than the allowable level for the semiconductor switching element. Thus, there has been a problem that the power supply operation of the semiconductor switching element may fail to be sufficiently achieved in some circumstances.